(1) Field of the Invention
This invention relates to the improvement of a solder alloy to be used for joining electronics components to printed circuit boards for electronics apparatus and the like. More particularly, this invention relates to a solder ally to assuage the phenomenon of fatigue rupture which occurs on a soldered joint exposed to a heat cycle stress for a long duration of time.
(2) Description of the Prior Art
A solder is a material primarily used for electrically or mechanically joining the plural number of parts, making use of metallurgical phenomenon of wetting. Among Sn--Pb solder alloys, one which has traditionally been used for assembling electronics apparatus or mounting electronics components is an eutectic solder with comparatively low melting point (Sn 63 wt %; Pb 37 wt %; melting point of approx. 183.degree. C.). The reason that such solder is used is because the compensation temperature is low for the heat resistance of electronics components, printed circuit boards, resin materials, etc.
An alloy soldered at a joint section is exposed to the stress omnidirectionally inflicted by compression, tensile, shearing or twisting produced by heat cycle which is caused by heating and cooling alternately repeated when turning on and off the electronics apparatus or by rise and fall of environmental temperature. The shifting of structure and re-crystallization are repeated within a solder alloy, off-setting the stress inflicted upon it. More particularly, the conspicuous growth of coarse crystallization is observed when the solder temperature rises almost as high as the solidus temperature or when it receives a severe cyclic stress. It is also noted that the stress caused by compression grows bigger, because the crystal turns harder and the shifting speed gets slower as a solder cools off. When the structure of a crystal has become no longer resistible to the stress, wrinkles occur over the inflicted area due to fatigue rupture and fissures appear with the growth of wrinkles. When the fissures start growing bigger, cracks can be observed outwardly.
An electrical resistance increases when cracks occur on a soldered joint, the temperature starts to rise with the flow of electric current generated when turned on, thereby lowering the mechanical strength. As a result, the cracks grow increasingly deep and wide to bring the primary purpose of joining materials to naught. Furthermore, since the electrical properties can also be deteriorated, it may jeopardize a proper function of an apparatus, or, for the worst case, it may generate spark or arc caused by the flow of electric current and induce ignition. More particularly, the matter of heat stress is of extreme importance in dealing with equipment driven by direct current or high voltage such as a new type of invertor apparatus, or car electronics which are to be exposed to severe circumstances, to say nothing of machineries and equipment used for daily life. It is indispensable to develop a soldering material excelling in the properties of heat resistance.
A solder alloy consisting of 90% of Sn, 9% of Cd and 1% of Zn, all in weight, with melting point of 238.degree. to 260.degree. C. is popular. Also popular is a solder alloy consisting of 95% of Sn and 5% of Sb in weight with melting point of 235.degree. to 240.degree. C. And further, Sn--Pb based solder added with either Cd, Ag, Bi, Cu, Au, Pd, Ni, Zn, In, As or Ca, Sn--Pb--Bi based solder, Sn--Pb--Sb based solder or a slight amount of Ag or Cu added Sn--Pb--Sb based solder is the alloy widely known (for example, Japan Publication Gazette SYO 49-21028, SYO 49-23986, SYO 53-113245, etc.).
Among the conventional solder alloys, for the solder which composition largely differs from that of an eutectic solder with melting point of 183.degree. C., a substantial change is required in its soldering temperature. Otherwise, there may occur the problems of work efficiency. Another problem involved is the fact that the thermal restriction by a heat resistance inherent in electronics components can not be dodged. Furthermore, there exists the problem with Sn--Pb--Bi based solder that the scope of application is limited because of its inferior creeping properties at high temperature and the deficiency in its shock resisting properties, although the solder with a low melting point suffice the conditions for the heat resistance temperature inherent in electronics components. More particularly, although the art to add Ga has already been practiced in some of the conventional arts mentioned above, there is no solid foundation to justify adding Ga nor specific guidance to specify the amount of an element to add. There exists no established standard available.
Another defect is that, depending on an element to be added, the impurities can be seen when a solder is dissolved, frequently causing poor soldering such as formation of bridges, icicles, etc. or producing solder dross in a large quantity. Particularly, no logical foundation to justify adding Cu is valuable, despite the fact that the art to add such element was introduced in the past. In the field of solder alloys, one which is widely known is Pb solder added with Cu for purpose of improving the flexibility inherent in Pb. On the contrary, when Cu is added to a solder alloy containing Sn, Sn--Cu intermetallic compounds are produced and the alloy becomes brittle. It is therefore generally recommended to avoid adding Cu.